1. Field of the Invention
This invention relates to diazotype coating compositions and reproduction materials. More specifically the present invention relates to 2-component diazotype material for both ammonia and amine development.
2. Brief Description of the Prior Art
The diazotype reproduction process is well known to the art and is described in great detail in "Light Sensitive Systems" by Jaromir Kosar, John Wiley & Sons, Inc., N.Y. 1965, and in "Reproduction Coating" by E. Jahoda, 4th Edition Andrews Paper & Chemical Co., Inc. Port Washington, N.Y. A more recently commercialized diazotype process for liquid amine development is described in U.S. Pat. Nos. 3,446,620 and 3,490,908. The two-component diazotypes for amine development are formulated for best performance in amine development (also called pressure development) machines which is in general detrimental to their performance in ammonia vapor development equipment. Diazotype materials for the amine development process, in general, do not develop fast enough in ammonia vapor development equipment and thus the trade is supplied with two different grades of diazotypes, one for ammonia and another one for amine development, for best performances. The present invention has equal applicability to both amine and ammonia development systems in that the compositions of the invention may be used to prepare diazotype reproduction materials that perform equally well in amine development machines and in ammonia vapor development processors.
Diazotype compositions for the two-component process comprise at least one diazo compound, at least one coupler and acidic stabilizers which are necessary to obtain a diazotype copy. Further components for such diazotype compositions may include:
1. development accelerators to minimize the necessary ammonia or amine concentration in the developing environment;
2. antioxidants to stabilize diazotype prints against discoloration under daylight exposure;
3. contrast controlling compounds; and
4. solubilizers which improve the compatibility of the various components in the sensitizing solution.
Sometimes such components fulfill more than one of these functions and often they have side effects which are detrimental to the quality of diazotype reproduction material.
It is one objective of this invention to produce improved diazotype compositions which overcome the above-mentioned shortcomings and the invention achieves that objective.
The rate of development of diazotypes for the dry process has become important recently as it is directly related to the necessary ammonia concentration in the developing machines. For environmental considerations it is of great interest to keep the ammonia concentrations at the lowest possible levels. Therefore, it is now important to provide for accelerated development of diazotypes. Desirably, acceleration is achieved without affecting print quality which is directly related to the degree of development. For example, print contrast, which can be defined by the Gamma value of the sensitometric curve of the diazotype material and can also be described as the optical density difference between full tone print line and print background, is easily affected by the degree of development. The rate of development of a diazotype will thus be a controlling factor for print contrast.
In the past, glycerine and glycols have been widely used as development accelerators because of their hydroscopic qualities. These compounds, however, tend to migrate into the sheet and, thus, their effect decreases during storage of the sensitized material. If they are applied in excessive concentrations, on the other hand, the sensitized paper loses its storage stability through precoupling which means formation of the print dye before the sensitized paper is used for the copying process. Another compound widely used as a development accelerator is hydroxyethyl-allyl-thiourea (described in U.S. Pat. No. 2,755,185). While this compound does not tend to migrate, it is rather expensive and only active with a very limited number of coupling components.
The most generally used antioxidizing agent in diazotype compositions is thiourea which strongly affects the shades of the print colors by, for example, shifting violet hues towards blue tones and increasing the brilliance of the print colors. Thiourea, however, is not of great value for development acceleration and thus tends to reduce print contrast with low ammonia concentrations in the development environment when full print development is not achieved. Moreover, there have been various literature references citing tumors in animals from ingestion of thioure and thiourea derivatives, thus making thiourea suspect of carcinogenic properties. Accordingly it would be advantageous to reduce or eliminate thiourea from diazo compositions.
The commercially available base paper for diazotype coatings is generally adjusted to an acid pH at the paper mill. The pH adjustment is made by addition of aluminum sulfate. Thus, diazotype base paper may contain various concentrations of aluminum sulfate which is a good stabilizer for diazotype compositions against premature decomposition of diazonium compounds and against precoupling. Its application however, is very restricted as its presence in diazotype compositions greatly reduces the rate of development. This effect is particularly pronounced when it is used with 2,3-dihydroxynaphthalene as a coupler component or with the corresponding acid sulfonated in the six-position. The aluminum ion seems to form a complex with these couplers to inhibit coupling.
A very useful component of diazotype compositions is zinc chloride which is beneficial for the shelf life of sensitized diazotype materials. It can also be used to adjust the rate of development and print contrast in many cases. Its application however, is often limited because it reduces the solubility of diazo compounds and the solution compatibility of diazo compounds with various couplers. The addition of solubilizers can improve tolerance for zinc chloride but an active solubilizing agent such as caffein tends also to render the print dye more water soluble, which is not desirable. I have now found that the use of a combination of 1,3-dimethylurea with urea or with thiourea in diazotype compositions, in replacement of thiourea, or instead of thiourea alone, produces improved diazotype coating compositions and reproduction materials by overcoming many of the above-described problems of the prior art. In particular, a great increase in the rate of development is obtained with an ensuing improved print color brilliance. Moreover, the solution compatibility of diazo compounds, couplers, and zinc chloride is greatly improved eliminating the risk of oily precipitations, in particular, at lower operating temperatures. I have also found that the above-detailed improved performance of diazotypes could not be obtained from the use of either of the compounds alone; i.e.; 1,3-dimethylurea or urea or thiourea alone. It appears that the combination with 1,3-dimethylurea generates a synergistic effect. I have also discovered that the brilliance of the print color was greatly superior when a mixture of 1,3-dimethylurea with urea or with thiourea was used instead of thiourea alone in the diazo coating composition. Appropriate print colors could be thus obtained with stoichiometric ratios of diazo and coupler while with thiourea or urea alone, 60 to 70% coupler excess is necessary to achieve similar results. There is of course an economic advantage in reducing the amount of coupler required. Thus, it is obvious that the application of this invention leads to more economical diazotype reproduction materials, as its compounds replace more costly components.
Contrary to all expectations, I have also found that aluminum sulfate can be used in diazotype compositions, with some couplers, with the result of an improved shelf life as well as an improved rate of development when 1,3-dimethylurea in mixture with urea or thiourea or both, is present. The improved toleration of aluminum sulfate with minimum adverse effects is outstanding even in the case of the 2,3-dihydroxynaphthalene couplers, in the presence of 1,3-dimethylurea in mixture with urea or thiourea.
The sensitivity to ultraviolet light of diazotypes reflects, for practical purposes, in their printing speed. While the printing speed is primarily reciprocally proportionate to the concentration of the diazo compound, various other compounds have a secondary influence on the printing speed. For example, by activating or deactivating the diazo compound, or by influencing its penetration into the base. Since the diazo concentration also controls the maximum obtainable print dye yield, it is desirable to formulate a diazotype composition that allows the fastest printing speed for a given diazo compound concentration. I have now found that diazotype compositions containing mixtures of 1,3-dimethylurea with urea or with thiourea produce diazotype copies with a faster printing speed than diazotype compositions without 1,3-dimethylurea.
The introduction of precoating in the diazotype (see TAPPI, Vol. 48, #8 pp. 55A-59A, August 1965) has greatly enhanced the print color brilliancy of diazotypes. The light-sensitive coating composition, however, has also a strong effect on the overall result regarding print brilliancy as well as color shade. I have discovered that diazotype compositions containing mixtures of 1,3-dimethylthiourea with urea or thiourea further enhance the brilliancy of the print dyes.
Other advantages of the compositions and reproducing materials of the invention will be described more fully hereinafter.